Nowadays, certain motor vehicles are able to communicate with equipment carried or worn by the users of these vehicles, such as an electronic ignition key or a smart phone.
Communication between a vehicle and such user equipment makes it possible for example to detect the presence of the user in a predetermined detection zone around the vehicle so as to activate certain functions as the user approaches or moves away from same. By way of example, these functions may be the locking or unlocking of the vehicle interior, or adjustments to equipment such as seats, rearview mirrors, climate control, etc.
In order to detect the presence of user equipment in the detection zone, the vehicle periodically, via an antenna, emits a radio signal containing a message known as the advertising message. When the user equipment receives this signal via its inbuilt antenna, it measures the strength thereof, this being referred to in the known way as RSSI (Received Signal Strength Indication) and communicates this strength value to the vehicle in an emission signal. The vehicle then uses this strength value to estimate the distance at which the user equipment is situated and thus determine whether or not the user is present in the detection zone.
However, it is found that the human body may have a negative impact on the performance of the antenna of the equipment, depending on where it is located in relation to the human body. Specifically, the tissues of the human body may absorb part of the radio signals emitted or received by the antenna of the user equipment, and cause a mismatch in the impedance of the antenna, thus leading to a loss in radiated power in the direction of the body of the user, which may for example be as much as 25 dB.
The antenna of the equipment may be oriented randomly and arbitrarily as it nears the vehicle. However, it is found that, with an antenna radiating pattern that is not omnidirectional, in combination with the impact that the human body has, it becomes absolutely essential to determine not only the position of the equipment in relation to the body of the user, but also the orientation of the user equipment with respect to the vehicle.
The loss of signal strength brought about by the position of the user equipment on the body reduces the RSSI value measured by the user equipment and sent to the vehicle, such that the distance values estimated may be erroneous, thus leading to errors in the detection of the presence of the user in the detection zone, something which therefore represents a major disadvantage.